The biochemical mechanisms operative in determining the rates of intracellular turnover of individual proteins are being analyzed, using a comparative approach with tyrosine aminotransferase (t1/2 equals 1.5 h) as typical of rapid turnover proteins and alanine aminotransferase (t1/2 equals 3 days) as a typical stable protein. Indications that rapid turnover involves interactions with other cellular components and not random proteolytic degradation are being pursued in attempts to identify and characterize such a component in turnover of tyrosine aminotransferase, and structural differences in the two enzymes that may contribute to differential turnover are being studied. When feasible comparable studies will be carried out on the cognate mRNAs, which exhibit comparable differences in rates of intracellular turnover.